程序代码：

void lookup_num()           //按学号查询
{
    char a;
    ifstream ist(filename.c_str());
    if(!ist)
    {
        cout<<"读取信息失败"<<endl;
        exit (0);
    }
    string line;
    string num1;
    string num;
    string oo="学号:";
    while(1)
    {
        cout<<"请输入要查询学生的学号:";
        cin>>num1;
        num=oo+num1;
        while(!ist.eof())
        {
            getline(ist,line);
            if(strstr(line.c_str(),num.c_str())!=NULL)
            {
                cout<<"该学生信息为:"<<line<<endl;
            }
        }
        cout<<"是否继续  1.继续查找    2.返回主菜单"<<endl;
        cin>>a;
        system("cls");
        if(a=='1') continue;
        if(a=='2') break;
    }
    a='0';
    ist.close();
}